Non-invasive presentation of internal body structures and organs by way of computed tomography or other x-ray imaging methods is a widely-used method in medical diagnostics. In such diagnostics a high contrast between bones and soft tissue can be obtained. However contrast between different soft tissues, because of small differences in absorption, is only suitable for diagnostics to a restricted extent.
Therefore contrast media are applied to increase the contrast of specific body structures or body fluids. These contain elements strongly absorbing x-ray radiation in order to obtain a high image contrast to the surrounding tissue with low absorption.
In radiological imaging by way of x-ray radiation, contrast media (CM) containing iodine are used nowadays for presenting body fluids, organs and pathological processes. Because of its absorption properties iodine is not the optimum element for contrast enhancement in x-ray diagnostics for tube voltages higher than 80 kV. This especially applies to CT, in which tube voltages of up to 140 kV are used nowadays. In the energy range of the x-ray radiation used the x-ray density of contrast media increases with the atomic number of the contrasting element. The use of contrast media of high atomic number is therefore especially suitable for CT, wherein, as well as the lanthanides, hafnium, rhenium, tantalum or tungsten are used as absorbing elements. In the use of each of these contrast media however, despite the high safety profile, undesired side-effects can occur.
Despite this, the largest proportion of contrast media currently used for x-ray diagnostics is based on iodine as the main x-ray attenuating component. Current device technology is optimized for this. In the choice of element for contrast media the element should exhibit an atomic number that is as high as possible, since the x-ray absorption increases strongly with the atomic number. The design of x-ray devices does not simply lie in the softest possible x-ray radiation producing the best contrast between water and the contrast medium.
The quality of the presentation or demarcation of two different tissue types or of tissue containing contrast medium from surrounding contrast-medium-free tissue can be described quantitatively by the contrast-to-noise ratio (CNR) between the two tissue types or between tissue with and without contrast medium. This is given by the CT value (also referred to as the attenuation value or HU value; HU=Hounsfield Unit) of the contrast medium and also the CT value of the neighboring contrast-medium-free tissue in relation to the image noise in this area.
The x-ray radiation used in radiological diagnostics is polychromatic, i.e. the wavelengths and thus the energies of the photons produced by an x-ray tube are not identical. The energy spectrum or photon spectrum of the x-ray radiation emitted by an anode is predetermined by the anode material and the tube voltage used. Tungsten anodes are used almost exclusively today in CT. Within the irradiated object the photon spectrum emitted is constantly changing since the absorption of x-ray radiation is energy-dependent and an overlaying with scattered photons occurs.
These interrelationships are complex and cannot be described by simple mathematical relationships. Thus the photon spectrum changes for example as a function of the irradiated volume, the penetration depth and the tissue composition. The x-ray density of a CM and thus the image signal is determined by the attenuation coefficient of the contrasting element, its local concentration, in combination with the photon spectrum at the point of physical interaction. Thus the actual irradiated thickness of the body of a patient is to be taken into account accordingly.
Because of the high probability of interaction between low-energy photons and tissue, the low energy components in the photon spectrum (in the range smaller than 50 keV), although they lead to a dose entry, they only make a small contribution to image generation. Thus, the use of photon energies of less than 50 keV leads with the same radiation dose to an increase in image noise.
Simulations show that, to obtain a high soft tissue contrast in CT under the condition of the minimum dose, photon energies ranging between 70 and 140 keV must be used. This is compared to contrast-medium-supported CT, in which the photon energies between 35 and 70 keV deliver the highest CNR-to-dose ratio. This contrast between native and contrast-medium-supported CT recordings is attributable to the spectral absorption characteristic of iodine.
The high absorption of iodine in the range of between 33 keV up to around 70 keV overcomes the unfavorable dose effect of low energy photons in such cases. With contrast media with elements of a higher atomic number, such as the lanthanoides, Hf, Ta or Re, the optimum energy range of the contrast-medium-supported CT shifts to higher energies in the range of between 60 and 140 keV and is thus practically identical to the optimum energy range for soft tissue contrast.
The amount of contrast medium administered to the patient depends on the planned examination and on the anatomy/physiology of the patient. For example a usual dose is 0.5 g of iodine per kg body weight for examinations of the parenchymatous stomach organs. The object is to achieve a contrast increase desired by the user in the relevant organs, in order thus to create a specific ratio of contrast and image noise (contrast-to-noise ratio CNR) in the CT images. Because contrast media can have undesired side-effects, the aim of many CT examinations, especially of older patients and patients with impaired kidney functions, is to minimize as far as possible the amount of contrast medium for the patient in order to achieve the objective of the examination.
According to the prior art, the amount of contrast medium administered to the patient is made dependent, as part of a contrast medium protocol, on an examination protocol (i.e. the type or objective of the intended examination, e.g. CT angiography, or examination of parenchymatous organs) and on the anatomy/physiology of the patient (height, weight, BMI, age etc.). The major parameters concentration, amount and flow rate of the contrast medium—depending on the planned examination and depending on patient characteristics such as height, weight etc., are mostly determined empirically by the doctor (e.g. 0.5 g iodine per kg of body weight for examinations of the parenchymatous organs). In the interim there have been also semi-automatic or fully automatic programs in accordance with the prior art which model the physiology and the circulation of the patient individually from parameters such as weight, size, heart performance, ejection rate of the heart etc., in order to obtain recommendations for a contrast medium protocol from said parameters.
The publication DE 102005052368 A1 from the same applicant discloses an x-ray system for creating diagnostic presentations of a patient with at least:                one x-ray tube for creating an x-ray bundle of x-rays with an energy spectrum for scanning the patient,        a detector for measuring the attenuation of the x-ray radiation as it passes through the patient,        an application unit for administering contrast medium for improving contrast in the x-ray imaging of the patient,        a control and processing unit for controlling the x-ray system and for creating the x-ray images of the patient with the aid of stored and executed computer programs, wherein        the x-ray system has an option for selecting different operating parameters at least in relation to the energy spectrum of the x-ray radiation used, wherein        the x-ray system has a selection device, which after direct or indirect specification of the parameters examination volume on one hand and tissue structure on the other hand, specifies at least a combination of contrast media and energy spectrum of the x-ray radiation for the examination with which an optimum contrast-to-noise ratio is achieved in the examination area for the smallest radiation dose and contrast medium load.        
Here the contrast-to-noise ratio (CNR) is defined as the optimization target, so that a combination of contrast medium and energy spectrum of the x-ray radiation optimized in this way is obtained. No minimization of a contrast medium load is possible with this however since the radiation dose for the body of the patient or the test item is included in the calculation which thus likewise cannot achieve a minimum.